Described below is a method for radio transmission of message symbols that are emitted as a wideband transmission signal by a UWB transmitter. UWB stands for ultra wideband (UWB) transmission, which is known per se and is also defined as the IEEE 802.15.4a standard in one form, for example. The wideband transmission signal is accordingly produced by pulse phase modulation (PPM—pulse position modulation) and therefore has a line spectrum having a multiplicity of modulated frequency lines. Pulse phase modulation is also called pulse position modulation in accordance with its designation above. Also described below is a system for radio transmission by ultra wideband transmission and a receiver for the system.
Pulse phase modulation is explained in more detail below with reference to FIG. 1. The figure shows a graph in which a control signal s for a UWB transmitter is plotted over time t, which control signal can control an oscillator for producing an electromagnetic radio signal, for example. During pulse phase modulation, the UWB transmitter can represent a succession 10 of message symbols 12, 12′ as a train of impulses or pulses 14, 14′ that are emitted in a manner shifted in time within a prescribed transmission pattern or time pattern 16. In this case, a time shift 20 for each pulse 14, 14′ is dependent on the respective message symbol 12 that is represented by the pulse 14, 14′. If a binary signal is transmitted, for example, then what is known as the alphabet of the possible message symbols from the transmitter can consist of the message symbols “0” (provided with the reference symbol 12 in FIG. 1) and “1” (reference symbol 12′). In order to transmit a succession of zeros and ones, a pulse 14 can then be transmitted for the message symbol “0” in each case at an instant that coincides with the time pattern 16. By contrast, the message symbol “1” can then be represented, in each case, by a pulse 14′ that is emitted in a manner shifted by one half of a pattern duration 22, as illustrated by the shift 20 in FIG. 1. The time pattern 16 defines a carrier frequency or pattern frequency frep, the reciprocal (1/frep) of which corresponds to the pattern duration 22 that is used to emit two successive pulses 14, 14′ for one and the same message symbol 12, 12′ when no additional coding measures, such as scrambling, are used. The time shift 20 for the message symbol “1” can then correspond to precisely 1/(2frep). The time signal for a single pulse may in each case be a square-wave pulse having a predetermined width, for example, or (as in the example from FIG. 1) else a differently shaped pulse 14, 14′. The actual electromagnetic transmission signal can also be shifted to a radio-frequency frequency range, e.g. between 1 and 10 GHz, by modulation, but this is not relevant to the explanation of the method.
The pulse-based transmission described is geared toward applications that require energy-saving inexpensive solutions. By way of example, this transmission allows the operation of wireless sensors with only a low energy requirement. The use of relatively narrow and hence wideband pulses additionally allows more robust data transmission, since cancellation effects as a result of multipath propagation (fading) have a much lower influence than in the case of narrowband transmission techniques
A disadvantage of pulse-based UWB communication however, is that a relatively complex reception circuit is required at the receiver end. A correlation receiver is needed for the individual pulse signals that is able to be used to identify the individual pulses and to recognize the position thereof in the time pattern. This is very complex in terms of circuitry.